1. Field of the Invention
The present invention relates to an image processing apparatus capable of controlling fixing conditions to fix toner on a sheet for processing multi-valued or binary image data. The invention also relates to an image processing method.
2. Related Background Art
For example, for an image processing apparatus, such as a color copying machine or color printer of electro-photographic method, there has been proposed printing on various kinds of paper sheets. There are, for example, board sheet, and OHT (Overhead Transparency), and further, those specially treated paper sheets, such as label sheets or sealing sheets. These sheets are different from a plain sheet in thermal conductivity, and in many cases, there is a need for more amount of heat for them than the plain sheet to execute fixing. The difference in amounts of heat needed for fixation is particularly conspicuous in a case of the color-printing engine that uses various toners.
Now, therefore, in order to give more amount of heat to a specially treated paper when fixing is executed, there have been known a method for increasing the amount of heat to be generated by a fixing device per unit time, and a method for reducing the speed at which the paper sheet passes a fixing device. Since the former allows the power dissipation to be increased, the latter is more often adopted in general. For example, it is practiced to enable an image processing apparatus to control the fixing conditions in accordance with the kinds of sheets on which toners are fixed.
However, when the fixing speed is determined depending only on the kind of sheet, it becomes necessary to reduce the fixing speed needed for an OHT to a ¼ of the fixing speed of a plain sheet. As compared with fixing toner for the plain sheet, the fixing toner for the OHT requires a larger amount of heat, and the printing productivity of the OHT is uniformly reduced corresponding to the kinds of sheets unavoidably.
In order to avoid the reduction of the printing productivity, if an amount of heat is given good enough for fixation without reducing the fixing speed, it is required for an image processing apparatus to consume an extremely large amount of electric power. It is not one of many optimum selections for an image processing apparatus to increase power dissipation even from the viewpoint of energy saving.
It is known that the amount of heat needed for fixation depends on the amount of toner on a sheet. The lesser the amount of toner, the smaller is the amount of heat that enables fixing to be executed. Also, the more the amount of toner, the larger is the amount of heat needed for the execution of fixing.
With the aforesaid relations between the toner amount and the heating amount needed for fixation, if the amount of toner is smaller, it should be possible to materialize fixing of toner without reducing the fixing speed actually even for a kind of paper sheet having unfavorable fixing capability, such as the OHT.
For fixing toner under the optimal fixing condition, it is necessary to acquire the amount of toner to be transferred to a sheet. Now, as a method for acquiring the fixing condition under which toner is fixed to a sheet, it is conceivable to work out a method for acquiring the fixing condition in accordance with the sum of the toner amounts used for one recording sheet, which has been obtained from image data.
However, in a case given below, it is not necessarily optimum that the fixing condition is determined on the bases of the sum of toner amounts used for one-sheet portion of a recording sheet. For example, such case is that the image data contain a mixture of characters and images to be represented on a recording sheet. In the case of the image data that contain characters and images, toners are biased to exist more often on the image portions. When the fixing condition is determined on the bases of the sum of toner amounts used for the one-sheet portion of a recording sheet, there is a possibility that the required amount of heat is not given sufficiently for the image portions.
In other words, if the toner distribution is biased for one-sheet portion of a recording sheet, it is not adequate from the viewpoint of the fixing capability of toner to the sheet that the fixing condition is determined on the bases of the sum of toner amounts used for one-sheet portion of the recording sheet.
Also, as another method for acquiring the fixing condition under which toner is fixed to a sheet, it is conceivable to acquire the fixing condition in accordance with the toner amount per pixel of the marked pixels that have been obtained from the image data. This method, however, may preset a case where the fixing condition thus acquired necessitates the supply of heat amount more than necessary for fixing toner.
For example, in a printer that processes multi-valued image data, if the density of a specific pixel is higher conspicuously than that of those surrounding it, the condition tends to be determined so as to supply heat in an amount needed for such one particular pixel. However, if the amount of toner needed for the pixels surrounding such one particular pixel is sufficiently small, the actually needed amount of heat for such particular pixel may be in some cases smaller than the thus acquired amount of heat, owing to the thermal conduction from the portions of the fixing device that corresponds to the surrounding pixels.
In other words, when the amount of heat needed for fixation is determined in accordance with the toner amount needed for one specific pixel, there may be present a case where heat is supplied in a wasteful amount eventually, and in terms of low power dissipation, the result may turn out to be unfavorable in some cases.
Also, in the case of color images, the toner amount may exert different influences given to fixing regarding toner of each color. For example, if the image processing apparatus is structured to use two-component toner for M, C, and Y, and one-component toner only for K, there may be encountered a problem that influences given to fixing are different per color, because the structure of each toner is different, among some others.